In many applications it is desired to know the Z-distance to a target object. A relatively accurate class of range or Z distance systems are the so-called time-of-flight (TOF) systems, many of which have been pioneered by Canesta, Inc., assignee herein. Various aspects of TOF imaging systems are described in the following patents assigned to Canesta, Inc.: U.S. Pat. No. 7,203,356 “Subject Segmentation and Tracking Using 3D Sensing Technology for Video Compression in Multimedia Applications”, U.S. Pat. No. 6,906,793 “Methods and Devices for Charge Management for Three-Dimensional Sensing”, and U.S. Pat. No. 6,580,496 “Systems for CMOS-Compatible Three-Dimensional Image Sensing Using Quantum Efficiency Modulation”, U.S. Pat. No. 6,515,740 “Methods for CMOS-Compatible Three-Dimensional image Sensing Using Quantum Efficiency Modulation”.
FIG. 1 depicts an exemplary TOF system, as described in U.S. Pat. No. 6,323,942 entitled “CMOS-Compatible Three-Dimensional Image Sensor IC” (2001), which patent is incorporated herein by reference as further background material. TOF system 10 can be implemented on a single IC 110, without moving parts and with relatively few off-chip components. System 100 includes a two-dimensional array 130 of Z pixel detectors 140, each of which has dedicated circuitry 150 for processing detection charge output by the associated detector. In a typical application, pixel array 130 might include 100×100 pixels 140, and thus include 100×100 processing circuits 150. (Sometimes the terms pixel detector, or pixel sensor, or simply sensor are used interchangeably.) IC 110 preferably also includes a microprocessor or microcontroller unit 160, memory 170 (which preferably includes random access memory or RAM and read-only memory or ROM), a high speed distributable clock 180, and various computing and input/output (I/O) circuitry 190. Among other functions, controller unit 160 may perform distance to object and object velocity calculations, which may be output as DATA.
Under control of microprocessor 160, a source of optical energy 120, typical IR or NIR wavelengths, is periodically energized and emits optical energy S1 via lens 125 toward an object target 20. Typically the optical energy is light, for example emitted by a laser diode or LED device 120. Some of the emitted optical energy will be reflected off the surface of target object 20 as reflected energy S2. This reflected energy passes through an aperture field stop and lens, collectively 135, and will fall upon two-dimensional array 130 of pixel detectors 140 where a depth or Z image is formed. In some implementations, each imaging pixel detector 140 captures time-of-flight (TOF) required for optical energy transmitted by emitter 120 to reach target object 20 and be reflected back for detection by two-dimensional sensor array 130. Using this TOF information, distances Z can be determined as part of the DATA signal that can be output elsewhere, as needed.
Emitted optical energy S1 traversing to more distant surface regions of target object 20, e.g., Z3, before being reflected back toward system 100 will define a longer time-of-flight than radiation falling upon and being reflected from a nearer surface portion of the target object (or a closer target object), e.g., at distance Z1. For example the time-of-flight for optical energy to traverse the roundtrip path noted at t1 is given by t1=2·Z1/C, where C is velocity of light. TOF sensor system 10 can acquire three-dimensional images of a target object in real time, simultaneously acquiring both luminosity data (e.g., signal brightness amplitude) and true TOF distance (Z) measurements of a target object or scene. Most of the Z pixel detectors in Canesta-type TOF systems have additive signal properties in that each individual pixel acquires vector data in the form of luminosity information and also in the form of Z distance information. While the system of FIG. 1 can measure Z, the nature of Z detection according to the first described embodiment of the '942 patent does not lend itself to use with the present invention because the Z-pixel detectors do not exhibit a signal additive characteristic. A more useful class of TOF sensor systems whose Z-detection does exhibit a signal additive characteristic are so-called phase-sensing TOF systems. Most current Canesta, Inc. Z-pixel detectors operate with this characteristic.
Many Canesta, Inc. systems determine TOF and construct a depth image by examining relative phase shift between the transmitted light signals S1 having a known phase, and signals S2 reflected from the target object. Exemplary such phase-type TOF systems are described in several U.S. patents assigned to Canesta, Inc., assignee herein, including U.S. Pat. No. 6,515,740 “Methods for CMOS-Compatible Three-Dimensional Imaging Sensing Using Quantum Efficiency Modulation”, U.S. Pat. No. 6,906,793 entitled Methods and Devices for Charge Management for Three Dimensional Sensing, U.S. Pat. No. 6,678,039 “Method and System to Enhance Dynamic Range Conversion Useable With CMOS Three-Dimensional Imaging”, U.S. Pat. No. 6,587,186 “CMOS-Compatible Three-Dimensional Image Sensing Using Reduced Peak Energy”, U.S. Pat. No. 6,580,496 “Systems for CMOS-Compatible Three-Dimensional Image Sensing Using Quantum Efficiency Modulation”. Exemplary detector structures useful for TOF systems are described in U.S. Pat. No. 7,352,454 entitled “Methods and Devices for Improved Charge Management for Three-Dimensional and Color Sensing”.
FIG. 2A is based upon above-noted U.S. Pat. No. 6,906,793 and depicts an exemplary phase-type TOF system in which phase shift between emitted and detected signals, respectively, S1 and S2 provides a measure of distance Z to target object 20. Under control of microprocessor 160, optical energy source 120 is periodically energized by an exciter 115, and emits output modulated optical energy S1=Sout=cos(ωt) having a known phase towards object target 20. Emitter 120 preferably is at least one LED or laser diode(s) emitting low power (e.g., perhaps 1 W) periodic waveform, producing optical energy emissions of known frequency (perhaps a few dozen MHz) for a time period known as the shutter time (perhaps 10 ms).
Some of the emitted optical energy (denoted Sout) will be reflected (denoted S2=Sin) off the surface of target object 20, and will pass through aperture field stop and lens, collectively 135, and will fall upon two-dimensional array 130 of pixel or photodetectors 140. When reflected optical energy Sin impinges upon photodetectors 140 in pixel array 130, photons within the photodetectors are released, and converted into tiny amounts of detection current. For ease of explanation, incoming optical energy may be modeled as Sin=A·cos(ω·t+θ), where A is a brightness or intensity coefficient, ω·t represents the periodic modulation frequency, and θ is phase shift. As distance Z changes, phase shift θ changes, and FIGS. 2B and 2C depict a phase shift θ between emitted and detected signals, S1, S2. The phase shift θ data can be processed to yield desired Z depth information. Within array 130, pixel detection current can be integrated to accumulate a meaningful detection signal, used to form a depth image. In this fashion, TOF system 100 can capture and provide Z depth information at each pixel detector 140 in sensor array 130 for each frame of acquired data.
In preferred embodiments, pixel detection information is captured at at least two discrete phases, preferably 0° and 90°, and is processed to yield Z data.
System 100 yields a phase shift θ at distance Z due to time-of-flight given by:θ=2·ω·Z/C=2·(2·π·f)·Z/C   (1)
where C is the speed of light, 300,000 Km/sec. From equation (1) above it follows that distance Z is given by:Z=θ·C/2·ω=θ·C/(2·2·f·π)   (2)
And when θ=2·π, the aliasing interval range associated with modulation frequency f is given as:ZAIR=C/(2·f)   (3)
In practice, changes in Z produce change in phase shift θ although eventually the phase shift begins to repeat, e.g., θ=θ+2·π, etc. Thus, distance Z is known modulo 2·π·C/2·ω)=C/2·f, where f is the modulation frequency.
But even with improved TOF systems such as exemplified by FIG. 2A, some portion of incoming optical energy received by a specific pixel 140 within array 130 will in fact be optical energy intended for another pixel within the array. What seems to occur is that within the pixel sensor array, some incoming optical energy reflects off shiny surfaces of the IC structure containing the array, typically reflective polysilicon traces and metal. The undesired result is that incoming optical energy that ideally would be sensed by a single pixel is instead reflected internally and is sensed by many pixels. Such internal reflections contribute to what is termed haze. This haze results from detection output signals being generated by pixels in the array other than the pixel that was intended to directly receive the incoming optical energy. As used herein, the term stray light will be used to refer to such misdirected optical energy, which manifests as a haze. In many applications, the magnitude of stray light is of no cause for concern. But in high performance TOF systems, such parasitic stray light and resultant haze can contribute to substantial measurement error in that wrong values of range Z will result from errors in detected phase-shift.
Thus there is a need for a mechanism to reduce if not substantially eliminate phase-shift errors in phase-type TOF systems due to parasitic stray light error.
The present invention provides mechanisms, implementable in hardware and/or software, to substantially eliminate such stray light error.